In the practice of conventional lithographic printing, it is essential to maintain sufficient water in the non-image areas of the printing plate to assure that image/non-image differentiation is maintained, that is, to assure that ink will transfer only to the image portions of the printing plate format. Many different dampening or water conveying systems have been devised and these systems can be referred to by consulting "An Engineering Analysis of the Lithographic Printing Process" published by J. MacPhee in the Graphic Arts Monthly, November, 1979, pages 666-68, 672-73. Neither the nature of the dampening system nor the nature of the dampening materials that are routinely used in the practice of high speed lithography are expected to place restrictions on the utilizing the teachings conveyed in this disclosure.
The dampening water in lithography is commonly supplied to the printing plate in the form of a dilute aqueous solution containing various proprietary combinations of buffering salts, gums, wetting agents, alcohols, fungicides and the like, which additives function to assist in the practical and efficient utilization of the various water supply and dampening systems combinations that are available for the practice of lithographic printing. Despite their very low concentrations, typically less than about one percent, the salts and wetting agents have been found in practice to be essential if the printing press system is to produce printed copies having clean, tint-free background and sharp, clean images, without having to pay undue and impractical amounts of attention to inking and dampening system controls during operation of the press. Apparently the dampening solution additives help to keep the printing plate non-image areas free of spurious specks or dots of ink that may be forced into those areas during printing.
It is well known in the art and practice of lithographic printing that ink is relatively easily lifted off, cleaned off, or debonded from most metallic surfaces, from most metal oxide surfaces and from virtually all high surface energy materials, such as the non-image areas of lithographic printing plates, by the action or in the presence of typical lithographic dampening solutions used in the printing industry. A similar phenomenon may occur when ordinary water or deionized water or distilled water is used without the dampening additives, but the debonding action of the water will generally be less efficient and will take place more slowly. In fact, lithographers have found that it is virtually impossible to produce acceptable lithographic printing quality efficiently or reproducibly using dampening water not containing the kinds of additives previously referred to.
Reference to R. W. Bassemir or to T. A. Fadner in "Colloids and Surfaces in Reprographic Technology", published by the American Chemical Society in 1982 as ACS Symposium Series 200, will relate that in the art of lithography the inks must be able to assimilate or take up a quantity of water for the lithographic process to have practical operational latitude. Apparently the ink acts as a reservoir for spurious quantities of water that may appear in inked images areas of the plate, since water is continuously being forced onto and into the ink in the pressure areas formed at the nip junction of ink rollers, dampening system rollers, and printing plates of the printing press. Whatever the mechanism might be, all successful lithographic inks when sampled from the inking system rollers are found to contain from about one percent to about as high as 40 percent of water, more or less, within and after a few revolutions to several thousand revolutions after start-up of the printing press. During operation of the press, some of the inking rollers must unavoidably encounter surfaces containing water, such as the printing plate, from which contact a more or less gradual build up of water in the ink takes place, proceeding eventually back through the inking train, often all the way to the ink reservoir. Consequently, the presence of water in the ink during lithographic printing is a common expected occurrence.
An important concept in this invention is recognition that all rollers of the purposefully foreshortened inking train of rollers in simplified ink systems must be either unreactive with water or not adversely affected by water or more precisely by lithographic dampening solutions which may have been transferred to the ink or that may otherwise be encountered by the inking rollers during routine operation of the printing press. If water can react or interact to displace the ink from any part of the inking rollers' surfaces, the transport or transfer of ink to the printing plate, thence to the substrate being printed, will be interrupted in that area, resulting in a more or less severe disruption in printed ink density and/or hue over some or all portions of the intended image areas and a concomitant loss of inking control. This invention provides means and material for avoiding that catastrophe.
In lithographic printing press inking roller train systems, it is typically advantageous to select materials such that every other roller of the inking train participating in the film splitting and ink transfer is made from relatively soft, rubber-like, elastically compressible materials such as natural rubber, polyurethanes, Buna N and the like, materials that are known to have a natural affinity for ink and a preference for ink over water in the lithographic ink/water environment. The remaining rollers are made usually of a comparatively harder metallic material or occasionally a comparatively harder plastic or thermoplastic material such as mineral-filled nylons or hard rubber. This combination of alternating hard or incompressible and soft or compressible rollers is a standard practice in the art of printing press manufacture. It is important to note, although it has not yet been explained, that the only practical and suitable metallic material the printing industry has found for use as the hard roller surface in lithographic inking systems is copper. Consequently, in the art of lithography, all metallic rollers for the inking system that will be subjected to relatively high dampening water concentration, namely those nearest the dampening system components and those nearest the printing plate, must and do have copper surface. Copper had been found long ago to possess consistent preference for ink in the presence of dampening water, unless it is inadvertently adversely contaminated. Means for cleaning or resensitizing contaminated copper surfaces towards ink are well known in the art of lithography. When any other practical hard metal surface such as iron, steel, chrome, or nickel is used in the place of copper, debonding of ink from the roller surface by dampening water may sooner or later occur, with its attendant severely adverse printed quality and process control problems. It is known that the relative propensity for debonding of ink from a surface depends in part, at least, upon the amount of water in the ink. Lithographic press manufacturers have found, for instance, that although ink can readily be debonded from hardened steel in the presence of modest to large amounts of water, small amounts of water in the ink, for example less than a few percent, generally may not cause debonding. Consequently, rollers near or at the incoming reservoir of fresh ink, that is near the beginning of typical multi-roller inking trains and therefore relatively far from the sources of water may be successfully used when manufactured from various hard, non-copper metals such as iron and its various appropriate steel alloys. The balance of the relatively hard rollers are commonly made using copper for the reasons just stated.
Although there has been speculation about the reasons for the advantageous properties of copper for use in inking rollers, it remains uncertain why copper tends to prefer ink over water. For the convenience of this disclosure, this property will be referred to as oleophilic meaning ink or oil loving and hydrophobic or water shedding. As indicated in this disclosure, certain of the rubber and plastic roller materials may be useful as the hard rollers in conventional, long train inkers. These, too, have the oleophilic/hydrophobic oil/water preference property, though perhaps for different scientific reasons than with copper.
In the case of metallic or polymeric rubber or plastic rollers, whether soft or hard, this oleophilic/hydrophobic behavior can be more or less predicted by measuring the degree to which droplets of ink oil and of dampening water will spontaneously spread out on the surface of the metal or polymer rubber or plastic. The sessile drop technique as described in standard surface chemistry textbooks is suitable for measuring this quality. Generally, oleophilic/hydrophobic roller materials will have an ink oil (Flint Ink Co.) contact angle of nearly 0.degree. and a distilled water contact angle of about 90.degree. or higher and these values serve to define an oleophilic/hydrophobic material.
I have found, for instance, that the following rules are constructive in but not restrictive for selecting materials according to this principle:
______________________________________ Best Water contact angle 90.degree. or higher. Ink Oil contact angle 10.degree. or lower and spreading. Maybe Water contact angle 80.degree. or Acceptable higher. Ink Oil contact angle 10.degree. or lower and spreading. Probably Not Water contact angle less than Acceptable about 80.degree.. Ink Oil contact angle greater than 10.degree. and/or non-spreading. ______________________________________
Another related test is to place a thin film of ink on the material being tested, then place a droplet of dampening solution on the ink film. The longer it takes and the lesser extent to which the water solution displaces or debonds the ink, the greater is that materials' oleophilic/hydrophobic property.
Materials that have this oleophilic/hydrophobic property as defined herein will in practice in a lithographic printing press configuration accept, retain and maintain lithographic ink on its surface in preference to water or dampening solution when both ink and water are presented to or forced onto that surface. And it is this oleophilic/hydrophobic property that allows rollers used in lithographic press inking roller trains to transport ink from an ink reservoir to the substrate being printed without loss of printed-ink density control due to debonding of the ink by water from one or more of the inking rollers.